Urine specific gravity (Usg) and urine osmolality (Uosm) are used routinely t o assess renal concentrating ability, but limited data on these variables are available for healthy dogs. Consequently, we studied the intra-and interindividual variations in Usg and Uosm in healthy dogs as well as the influence of age and gender on these variables. Dogs were selected for health and anestrus in female dogs through the use of a detailed questionnaire. Eighty-nine owners collected morning and evening urine samples from their dogs on 2 consecutive days. In 8 dogs in which the Uosm of different samples varied more than 50%. owners collected urine for 24 hours at 2-hour intervals during the day and at 4-hour intervals at night. The possible effect of changes in adrenocortical function with age was assessed by measurements of urinary corticoid/creatinine (C/C) ratios.Among all samples, Uosm ranged from 161 t o 2,830 mOsm/kg and Usg from 1.006 t o > 1.050. In the morning, Uosm (1,541 2 527 mOsm/kg, range 273-2,620 mOsmlkg1 and Usg (1.035 t 0.010, range 1.009-> 1.050) were higher than in the evening (Uosm 1,400 t 586 mOsm/kg, range 161-2,830 mOsm/kg; Usg 1.031 t 0.012, range 1.006-rinary concentrating capacity depends on the ability of U the hypothalamic osmoreceptors to respond to changes in plasma osmolality, the ability of the atrial and carotid bifurcation baroreceptors to respond to changes in blood pressure or blood volume, and the release of the antidiuretic hormone vasopressin (VP) from the neurohypophysis. In addition, medullary hypertonicity must be generated and maintained, and there must be an adequate number of functional nephrons with an appropriate response to VP.'.' In the healthy dog, urine osmolality (Uosm) values as low as 50 mOsm/kg are reached after infusion of large quantities of water, that is, in the absence of VP? In states of dehydration, urine specific gravity (Usg) and Uosm are increased to as much as 1,076 and 2,738 mOsm/kg, re~pectively.~ Urine specific gravity and Uosm are used to assess the ability of the renal tubules to concentrate or dilute glomerular filtrate.' Randomly collected urine samples from normally hydrated dogs are reported to have Usg values ranging from 1.015 to 1.045,'-'' whereas Uosm values usually vary between 500 and 1,200 mO~m/kg.'."-'~ In some dogs, Uosm varies widely during the day, with an intraindividual coefficient of variation approaching the interindividual coefficient of variation. This may be regarded as a biologic variation but also could represent an early undiagnosed clinical abnormality.

Physical and emotional stresses are known to increase the production and secretion of glucocorticoids by the adrenal cortex in both humans and experimental animals. The urinary corticoid:creatinine (C:C) ratio is increasingly used as a measure of adrenocortical function. In this study we investigated whether a visit to a veterinary practice for vaccination, a visit to a referral clinic for orthopedic examination. or hospitalization in a referral clinic for I .5 days resulted in increases of the urinary C : C ratio in pet dogs. In experiment I. owners collected voided urine samples from 19 healthy pet dogs at specified times before and after taking the dogs to a veterinary practice for yearly vaccination. In experiment 2, 12 pet dogs were evaluated in a similar way before and after an orthopedic examination at a referral clinic. In experiment 3, 9 healthy pet dogs were hospitalized for 1.5 days and urine samples were collected before, during. and after this stay. Basal urinary C:C ratios in all experiments ranged from 0.8 to 8.3 X 10 6. In experiment 1, the urinary C : C ratio after the visit to the veterinary practice ranged from 0.9 to 22.0 X 10. ' . Six dogs had a significantly increased urinary C:C ratio (responders), but in 5 of these dogs the ratio was 510 X lo-". In experiment 2, 8 of 12 dogs responded significantly with urinary C:C ratios ranging from 3.1 to 27.0 XIn experiment 3, 8 of 9 dogs had significantly increased urinary C : C ratios. ranging from 2.4 to 24.0 X in some or all urine samples collected during hospitalization. In 4 dogs urinary C : C ratios 12 hours after hospitalization were still significantly higher than the initial values. Thus, a visit to a veterinary practice. an orthopedic examination in a referral clinic. and hospitalization can be considered stressful conditions for dogs. A large variation occurs in response, and in individual dogs the increases in urinary C: C ratios can exceed the cutoff level for the diagnosis of hyperadrenocohcism. Therefore, urine samples for measurement of the C : C ratio in the diagnosis of hyperadrenocorticism should be collected in the dog's home environment, to avoid the influence of stress on glucocorticoid secretion.

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